Traditionally, there have been an optical divider, an optical demultiplexer, and an optical multiplexer, in which three optical fibers serving as a waveguide are disposed on a single plane and fused and drawn so as to divide, demultiplex, or multiplex incident light.
For Example, Patent Document 1 discloses an optical fiber coupler in which two optical fibers whose propagation constants are substantially the same and a single dummy optical fiber are arranged, fused and drawn. This realizes an optical fiber coupler for equal division, having a broad band wavelength characteristic. Further, Patent Document 2 discloses an optical fiber coupler in which three optical fibers including an optical fiber whose normalized frequency and/or a fiber diameter is/are different from those of the other optical fibers, which are arranged, fused and drawn. This realizes an optical fiber coupler by which two optical signals of 1.3 μm and 1.55 μm incident to a single port on the input side are demultiplexed into optical signals of 1.3 μm and 1.55 μm at the output side, and the optical signal of 1.3 μm is equally divided.
Further, Patent Document 3 discloses a optical fiber optical multiplexer in which three optical fibers including an optical fiber whose core diameter is greater than those of the others, are arranged, fused and drawn. This realizes an optical fiber optical multiplexer capable of multiplexing optical signals of a single wavelength with a low loss.